Environmental enhancement through use of recycled aggregate concrete in a two-stage mixing approach

As a high priority of waste management and recycling by the Hong Kong government, Recycled Aggregate (RA) has been used in various construction applications, mainly as sub-grade, roadwork, and unbound materials. However, higher-grade applications are rare. The major barrier encountered is the variation of quality within RA, which causes lower strength, resulted from crystallization of Recycled Aggregate Concrete (RAC). Therefore, the objective of this study is to examine the crystallization of RAC in a Two-Stage Mixing Approach. Following are the five areas of interest: (i) investigate the waste problems in construction activities; (ii) examine the crystal development on the hydration of cement paste; (iii) develop a two-stage mixing approach (TSMA) for improving the performance of RAC; (iv) explore the crystallization of TSMA in comparison with the Normal Mixing Approach (NMA) through use of Differential Scanning Calorimetry (DSC); and (v) verify the results obtained from DSC analysis with those obtained from compressive strength testing. This study adopted 0, 20, and 100% RA substitution in virgin aggregate and measured by DSC and compressive strength on both TSMA and NMA. TSMA uses only half the water for mixing, forming a thin layer of cement slurry on the surface of RA that will permeate into the porous old cement mortar and fill old cracks and voids in the pre-mix process. The restuts from DSC analysis clearly demonstrated that TSMA can give a better crystallization of CaO·SiO₂·H ₂O [CSH] and Ca(OH)₂[CH]. The optimal situation occurs on 20% RA substitution in virgin aggregate, balancing the advantages of each, a finding supported by the results from compressive strength testing. Therefore, TSMA is a superior methodology and opens a wider application for the use of RAC.